i too like to use 2 balls of yarn.
but here is another trick.
look at schematic--how big is the cast on?
front of sweater (11 to 15 inches?)
back of sweater (20 to 30 stitches?)
Hat (19 to 20 stitches?
you need pi times width. (or 3 times width and a bit more.)
(because you need to have enough yarn to wrap around a needle (a circle right!?)
20 inch (sweater back?) more than 60 inches, less than 72.
do you know how long a space from finger tips to nose? take a tape measure and measure outstretched arm fingertip to nose.
(on me, its 34 inches.) depending on your height/build it will be be between 30 to 36 inches.
so if i am casting on for 25 inches, i need more than 75 inches ( and 34 +34 =68, so 2 full lengths + 1/2 (anther 17 inches) is about right--I need more than 2 full lengths, but less than 93 (3 full lengths from nose to fingertips)--it is not perfect but close.. (and rarely too short!)
and Pi times centimeters words too.. (d'oh) and measuring your arm to nose works the same!
works well with big cast ons too.. making a blanket and its going to be 60 inches wide? you need over 200 inches for cast on!
(more it you are doing cables or anything that compresses the knitting)